1. Field of the Invention
The invention relates to a fuel injection system for internal combustion engines.
2. Description of the Prior Art
The subject of German Application DE 19853103.6 is distinguished by a fuel metering unit which is capable of metering exactly the desired fuel quantity in the applicable operating state of the engine to the high-pressure pump of the common rail (CR) system. By this kind of exact metering on the low-pressure region of the required fuel quantity to the high-pressure pump, compressed overflows are averted from the very outset, which leads to improved efficiency and thus to fuel economies.
In the subject of the above-referenced German Application DExe2x80x9419853103.6, radial control openings in the valve housing and an axial opening that leads to the interior of the sleevelike valve piston are essential structural elements. Under these structural preconditions, the aforementioned advantages can be achieved by means of two alternative flow principles: In one alternative, the axial opening connects the interior of the valve piston to a prefeed pump of the fuel injection system, and the radially oriented control openings of the valve housing operatively communicate hydraulically with the low-pressure region of the high-pressure pump. In the other alternative, this flow principle is reversed; now the axial opening communicates hydraulically with the low-pressure region of the high-pressure pump, while the control openings communicate with the compression side of the prefeed pump and thus form the inflow into the metering unit.
The object of the present invention is to further optimize this latter, xe2x80x9creversedxe2x80x9d flow principle.
According to the invention, one advantage is that the valve piston in the valve housing is hydraulically centered over 360xc2x0, especially whenever a suitable annular conduit is embodied in the valve housing. In this way, sliding with little hysteresis and hence optimal sliding of the valve piston in the valve housing is made possible.
By disposing the control openings at equal angular spacings on the circumference of the valve piston, preferably in diametrically opposed pairs, an optimal hydraulic flow force compensation is attained.
From a production standpoint as well, the invention proves to be advantageous, since it is in fact simpler and less complicated to machine the control openings (for instance by laser cutting and ensuing deburring) in the valve piston than would be the case in what is already a cost-intensive valve housing. The variation from one application to another in the control openings accordingly takes place in the valve piston and not in the cost-intensive valve housing.